LED lighting is evolving from fixed output device towards intelligent controllable lighting. This intelligent lighting may include remote dimming functionality, color mixing ability, as well as data collection functionality (for example from on-board sensors such as proximity sensors and temperature sensors). The light output may thus be controlled in various ways to provided value added features.
To make a light unit remotely controllable, normally a main controlling unit (normally microcontroller unit or MCU) is required to be integrated into the electronic control gear (ECG) or driver. The driver then requires an auxiliary power supply provided to the MCU so that it can continuously perform communication and control tasks. This power supply needs to be continuously available.
During a time when the light unit is turned off, with only the MCU and associated communication and control circuit operational, this auxiliary power is referred to as the standby power. This auxiliary power can be derived from the power supply to the light unit fixture.
However, when the light unit is turned off, it is no longer simply possible to tap an auxiliary supply from a transformer used to power the light unit. Instead, another power source is needed. This results in inefficiency and additional circuit cost. There are directives which impose a standby power consumption requirement for the controlling apparatus, for example a maximum 0.5 Watt standby power requirement for a lighting control unit.
There is therefore a need for a low standby power consumption and also for a low cost auxiliary power solution for use in an intelligent LED light solution. The need for such a solution has been recognized. For example, US 2014/0346874 disclosed a known standby power solution, and the circuit is shown in FIG. 1.
The circuit comprises a front end fly back stage 1 which generates a bus voltage and a back end buck stage 2 which regulates the output current. The circuit topology thus consists of two stages. The first stage delivers a constant voltage output and the second stage delivers a constant current output.
The auxiliary power comprises two parts by dividing auxiliary circuits into those which need to be active during a standby mode and those which do not. The two power supplies operate in parallel mode. The auxiliary power efficiency is low because a linear regulator is always active. This circuit also has a dedicated constant current control logic block with added cost and power loss.
US20110080110A1 also discloses a two-stage LED driver with a first stage that regulates its output voltage and a second stage that converts the output voltage of the first stage into regulated current or voltage. The first stage powers an auxiliary supply either from a secondary winding or from its output voltage.
WO2015056161A1 discloses selectively providing power to a control unit using different voltages at different stages of the overall drive unit. The auxiliary supply obtains power either from the bus voltage or from the cathode of the LED.